78 FLORIDA STATE HORTICULTURAL SOCIETY, 1970 crements and losses through evapotranspiration including types of systems, power units and pumps. Univ. Fla. Ag. Eng. Mimeo Rept.. 65-9. can be maintained. Tailored to the individual 10. , and R. E. Choate. 1968. Selection of pumps and power units for irrigation systems inFlorida. situation (soil type, drainage, rooting depth, Fla. Ag. Ext. Serv. Circ. 330. etc.), this method takes much of the "guesswork" 11. Koo, R. C. J. 1963. Effects of frequency of irriga tion on yield of orange and grapefruit. Proc. Fla. State out of the irrigation system. Hort. Soc. 76: 1-5. 12. 1969. Evapotranspiration and soil moisture determination as guides to citrus irrigation. Proc. Conclusion First Int'l. Citrus Symposium, Univ. of Calif., Riverside 3: 1725-1730. 13. Koo, R. C. J., and G. T. Hurner. 1969. Irrigation re In conclusion, the irrigation operation, inte quirements of citrus grown on Lakewood fine sand. Proc. Fla. State Hort. Soc. 82: 69-72. grated with drainage in the total water manage 14. , and A. A. McCornack. 1965. Effects ment system, is an expedient to economic produc of irrigation and fertilization on production and quality of 'Dancy' tangerine.Proc. Fla. State Hort. Soc. 78: 10-12. tion of citrus only when all aspects of the opera 15. Muma, M. H. 1970. Environment and control of in jurious insects and mites in Florida citrus groves. Cit. Ind. tion are properly planned. The risks of high Mag. 51(6): 4, 26. costs, with no value for the operation, are so 16. Phillips, R.L. 1969. Performance of closely spaced trees. Proc. Fla. State Hort. Soc. 82: 48-51. great that the use of the limited resources of 17. Reitz, H. J., and W. T. Long. 1955. Water table fluctuations and depth of rooting of citrus trees in the capital should be carefully budgeted. In the final Indian River area. Proc. Fla. State Hort. Soc. 68: 24-29. analysis, however, irrigation can be a very eco 18. Reuss, L. A. 1969. Yield response and economic feasibility of sprinkler irrigation of citrus, central Florida. nomic and desirable production practice. Univ. Fla. Econ. Mimeo Rept. EC 69-10. 19. , and D. S. Harrison. 1969. Inputs and costs of selected sprinkler irrigation systems for citrus LITERATURE CITED in central Florida. Univ. Fla. Econ. Mimeo Rept. EC 69-8. 20. Schirard, J. B. 1969. In-place rejuvenation. Cit. Ind. 1. Bryan, O. C. 1962. The response of young trees to Mag. 50(10) : 7. lime mixed with the soil profile in Leon-lmmokalee fine 21. Schmidling, J. 1970. They're taming the water sands. Cit. Ind. Mag. 43 (9): 9-10. table on flatwood soils. Fla. Gr. and Rancher (N. Ed.) 63(1): 2. Calvert, D. V., R. C. J. Koo, and H. W. Ford. 1967. 8, 25. Flood irrigation studies with citrus. Proc. Fla. State Hort. 22. Sites, J. W. 1947. Internal fruit quality as related to Soc. 80: 79-85. production practices. Proc. Fla. State Hort. Soc 60: 55-62. 3. , and H. J. Reitz. 1965. Salinity of 23. , L. C. Hammond, R. G. Leighty, W. water for sprinkle irrigation of citrus. Proc. Fla. StateHort. O. Johnson, and K. D. Butson. 1964. Information to con Soc. 78:73-78. sider in the use of flat woods and marshes for citrus. Fla. Ag. 4. Choate, R. E., and D. S. Harrison. 1968. Irrigation Expt. Sta. Circ. S-135-A. by the accounting method. Univ. Fla. Ag. Eng. Mimeo 24. , H. J. Reitz, and E. J. Deszyck. 1951. Rept. 68-5. Some results of irrigation research with Florida citrus. 5. Driscoll, P. J. 1969. Renovating old citrus groves. Proc. Fla. State Hort. Soc. 64: 71-79. Cit. Ind. Mag. 50(3) : 22-24. 25. Tucker, D.P.H., and R. L. Phillips. 1967. Tree size 6. Ford, H. W. 1954. The influence of rootstock and control in citrus groves of the future. Cit. World 4(6) : 16-17, tree age on root distribution of citrus. Proc. Am. Soc. 26. Hort. Sci. 63: 137-142. 26. Young, T. W. 1953. Soil moisture relations in the 7. . 1954. Root distribution in relation coastal citrus areas of Florida. Fla. Ag. Expt. Sta. Bui. 526 to the water table. Proc. Fla. State Hort. Soc. 67: 30-33. 27. Ziegler, L. W. 1955. Irrigation studies with Marsh 8. . 1964. The effect of rootstock, soil grapefruit on Lakeland fine sand in central Florida. Proc. type, and soil pH on citrus root growth in soils subject Fla. State Hort. Soc. 68: 39-41. to flooding. Proc. Fla. State Hort. Soc. 77: 41-45. 28. 1970. The evaluation of soils for 9. Harrison, D. S. 1965. Citrus irrigation in Florida, citrus production in Florida. Cit. Ind. Mag. 51(1): 14-15.
RANGPUR LIME AS A CITRUS ROOTSTOCK IN FLORIDA
Mortimer Cohen because of its susceptibility to foot rot, exocortis, and xyloporosis. Observations made in experi Indian River Field Laboratory mental plantings illustrate the productivity, high Fort Pierce quality and good survival record of orange and grapefruit trees on this stock with and without Abstract exocortis. Exocortis often appears to induce a condition of dwarfing which could be desirable. 'Rangpur' lime (Citrus limonda Osbeck) is the The generally good experience of growers in 17 main rootstock used in Brazil but it has had only young commercial groves with a number of scion limited acceptance as a citrus rootstock in Florida varieties is described. The setting-out of new, selected, small plantings on 'Rangpur' lime root- Florida Agricultural Experiment Station Journal Series stock is justified. No. 3748. COHEN: RANGPUR FOR ROOTSTOCK 79
Introduction this country were propagated from budwood which carried this virus. The discovery that Constantly changing conditions in the citrus exocortis is a virus disease was made in 1949 industry keep the citrus grower on a never-end (2). Knowledge that 'Rangpur' lime is suscep ing quest for new rootstocks. The perfect root- tible to exocortis came in 1952 (14) and it was stock will, perhaps, never be found but any only in 1964 (3) that a practical and rapid means grower can cite the deficiencies of the stocks of indexing for exocortis was discovered. Even currently in most common use in Florida. The more recent are the concepts that the biggest advent of tristeza could mean calamity to groves trees are not necessarily the best and that the on sour orange (Citrus aurantium L.) stock and ability of exocortis and other viruses to induce a new problem, young tree decline, threatens new the production of small trees was a proper sub groves on rough lemon (C. jambhiri Lush.). A ject for serious study and consideration (1, 5, host of other factors make * Cleopatra' mandarin 12). Very important also is an understanding (Citrus reshni Hort. ex Tan.) and sweet orange of the existence of different strains of exocortis (Citrus sinensis (L.) Osbeck) less than perfect. virus whose effect on trees ranges from negligible Some other stocks, like trifoliate orange (Pon- to drastic (4). Under these circumstances it is drus trifoliata (L.) Rafinesque), 'Carrizo' not surprising that few studies of the interaction citrange (Poneirus X C. sinensis) and Citrus of exocortis strain, rootstock and environment macrophylla Wester are well worth trying on an are available. experimental basis in small plantings but ex In 1960 Sinclair and Brown (19) reported on perience with them is limited. This paper deals the effect of exocortis disease on 9-year old navel with another stock which is not well known to orange (Citrus sinensis (L) Osbeck) trees on the grower but about which information is now 'Cleopatra' mandarin, sweet orange, trifoliate becoming available. orange and 'Rangpur' lime. They found both 'Rangpur' lime is not unknown in the world yield and trunk circumference reduced for all scene. It is the most important rootstock of cit combinations. Olson and Shull (15) studied 12- rus in Brazil (13) and is also used in Argentina. year old 'Valencia' orange scions with and with It apparently originated in India (18) and out exocortis and xyloporosis virus on various seedling plants can be found in almost every rootstocks. Their data for trees on 'Rangpur' citrus-growing country. 'Rangpur' lime as a and other mandarin-limes showed that trees rootstock has been the subject of contradictory carrying the viruses were greatly reduced in size reports which often had their basis in the and yield as compared with uninfected trees. In changes in character of this stock when infected contrast, exocortis and xyloporosis infection pro with different strains of exocortis and xyloporo- duced relatively little depression of growth and sis. Results of some experiments are presented yield of trees on 'Cleopatra' mandarin, rough here which may help resolve some of the con lemon and sour orange. tradictions. Also given is a summary of grower In a study of the ability of citrus roots to experience with 'Rangpur' rootstock in 17 situa survive flooding, Ford (9) had one test involving tions on more than 400 acres of grove. 'Rangpur' lime. He found that 'Rangpur' and It will be seen that not only is 'Rangpur' lime 'Carrizo' were somewhat less damaged than a promising rootstock for Florida when used 'Milam' lemon and rough lemon and distinctly under virus-free conditions but controlled and better than Poneirus trifoliata, sweet orange, selected virus inoculation of this stock may give sour orange and 'Cleopatra' mandarin. the grower unprecedented flexibility in control of tree size and of the solids content of the In some reports 'Rangpur' was found to in juice. duce as much cold susceptibility as rough lemon (7, 11) while in others trees on 'Rangpur' were observed to be somewhat more resistant to cold Characteristics op 'Rangpur' Lime as a than those on rough lemon (8, 10). Despite the Rootstock differences reported it is evident that trees on The major difficulty involved in evaluating 'Rangpur' must be considered relatively suscep 'Rangpur' lime as a citrus rootstock lies in its tible to cold injury. susceptibility to exocortis disease and the fact Mixed reports are also found with respect that most of the older rootstock experiments in to the susceptibility of 'Rangpur' lime to foot 80 FLORIDA STATE HORTICULTURAL SOCIETY, 1970
rot disease. Moreira (13) describes 'Rangpur' Growers on the east coast of Florida, known as being "fairly resistant to Phytophthora foot to have used 'Rangpur' lime as a rootstock were rot" and Cooper et al (7) describes foot rot as questioned about their plantings. Table 4 is a a problem of other stocks but do not mention it summary of their reports. in connection with 'Rangpur' lime. Nevertheless growers' observations listed in Table 4 indicate Results that foot rot has been a serious problem of 'Rangpur' in young plantings in Florida. Four rootstock experiments, all with sweet orange scions, are summarized in Table 1. Trees
Method in Experiment 1 are free of exocortis (and psorosis and xyloporosis), but trees in the other To document the performance of trees on 3 experiments listed in Table 1 carry a mild 'Rangpur' lime rootstock, tabulated summaries strain of exocortis. Despite this difference in are given in Tables 1 and 2 of observations of exocortis status the average yield per tree of trees on 'Rangpur' and, for comparison, on rough trees on 'Rangpur' was close to or better than lemon and sour orange. Data for these tables has the yield of trees on rough lemon and distinctly been selected from 7 rootstock experiments. Four better than yield of trees on sour orange. Cur experiments, which are now 20 years old, have rent yield data show no indication of change in been described previously (6, 17). Results from this situation. A similar trend in tree size is three other experiments which are 10 years old shown by average canopy diameter. With re have not been published before. The effects of spect to Brix of juice (total soluble solids), trees introducing different strains of exocortis virus on rough lemon and sour orange are at opposite into 'Marsh' grapefruit trees on various root- ends of the scale, as expected. Fruit from trees stocks are being studied in a 5-year old experi on 'Rangpur' lime are distinctly higher in Brix ment. The experimental unit consists of 6 single than those from trees on rough lemon but are tree replications. Results from this experiment closer to the rough lemon rating than the rating for 3 rootstocks are extracted in Table 3. All for sour orange. In Experiment 2 one tree on rootstock experiments described are in St. Lucie 'Rangpur' lime is missing, probably due to foot County, Florida. rot. The missing tree on 'Rangpur' in Experi-
Table 1. Site, size, productivity, quality, and survival of orange trees carrying no exocortis or a mild strain of the virus.
Average ^Average
yield canopy Expt. Soil Virus tree diameter /AN no. Variety Age type strain Rootstock (boxes) (feet) ,<" Tree survivalv '
1. Pineapple 10 .Felda None(2> Rangpur 2.7 12.3 10 .16 10/0/0 II Rough lemon 3.1 13.5 10 .00 10/0/0 II Sour orange 1.9 11.5 10 .89 10/0/0
2. Valencia 10 Felda Mild Rangpur 1.6 12.2 11 .94 10/1/0 it Rough lemon 1.3 13.7 11 .61 10/0/0 ii Sour orange 0.8 12.4 12 .77 10/0/0
3. Valencia 20 Wabasso Mild Rangpur 2.0 14.8 11 .60 14/1/1 * (3) Rough lemon 2.0 15.2 11 .56 14/0/4 ? (3) Sour orange 1.2 14.1 12 .43 14/0/0
4. Valencia 20 Park- Mild Rangpur 2.7 18.0 11 .68 5/0/0 wood ? (3) Rough lemon 3.0 20.5 11 .16 8/0/1 ? (3) Sour orange 2.1 17.2 12 .46 6/0/0
(1) Average ?f at lea8t 5 years of data for 10 vear old trees and 14 years of data for 20 year old trees. Old-line*bud source; negative for exocortis by citron tests. Two sources of budwood used, one with a mild and one with a strong strain of exocortis. On exocortis tolerant stocks like rough lemon it is impossible, without indexing, to determine the strain carried. Such indexing has not yet been done. Tree survival: Number of trees planted/number dead or missing/number in decline. Formerly classified as "Leon". COHEN: RANGPUR FOR ROOTSTOCK 81 ment 3 was killed by lightning. All trees in Table 3. Productivity, size and quality of 5-year old 'Marsh1 decline, 4 on rough lemon and one on 'Rangpur', grapefruit trees inoculated when one year old with specified strains of exocortis. show symptoms of citrus blight.
The 3 grapefruit (C. graovdis (L.) Osbeck) yield/ rootstock experiments covered in Table 2 carry l££°ck_ Inoculated (boxes) £feet; strong strains of exocortis. Average yield per strong 3.4 376.9 0.95 8.98 388.2 0.95 8.57 tree is approximately the same for trees on all 388.2 0.98 8.72 363.0 0.98 9.09
3 rootstocks in Experiment 1. It should be noted i strong 401.8 95 0.98 8.20 none that trees on rough lemon and sour orange have none*-*' 407.4 7 7.98 a larger canopy diameter than those on 'Rang- 0 338.3 7 8.21 5 390.4 1.02 8.92 pur' lime. Experiment 2 consists of the same ,0 388.2 1.03 8.71 358.7 1.08 8.57 types of trees as Experiment 1 but is located on 395.0 1.05 8.68 a hammock-type soil (Parkwood) instead of the * ' Two-year average. '^J "or fruit sampled about flatwoods type (Wabasso, formerly classified as December 1. (3) Non-exocc ! Inoculated. "Leon") used in Experiment 1. Yield of trees on rough lemon is almost twice as great as trees lemon were affected as on 'Rangpur' (Table 2). on 'Rangpur' in Experiment 2 but the grove area Trees on sour orange are unaffected. Table 2 occupied by trees on rough lemon is more than shows additional trees on rough lemon in decline twice as large as that occupied by trees on in Experiments 2 and 3. The 2 dead trees in 'Rangpur'. The situation in Experiment 3, Table Experiment 3 on 'Rangpur' succumbed early in 2 with respect to yield and tree area is similar the life of the planting. Cause of death was foot to Experiment 2. Brix of fruit juice of the 3 rot in one case and was not clear in the other. experiments involving strong strains of exocortis The first results from an experiment which is highest for trees on sour orange but fruit from has not been described previously are presented 'Rangpur' lime is closer in Brix to the high in Table 3. The full experiment involves 16 standard of sour orange than the low level reg rootstocks for a nucellar source of 'Marsh' istered for fruit from trees on rough lemon. Grapefruit. Trees on many stocks are divided, Within the past year there has been some de like the 3 shown, into 4 groups. Trees in 3 terioration in the health of the trees in Experi of the groups were inoculated as shown when one ment 1, Table 2. The cause of the condition is year old. The fourth group was uninoculated. not known but 3 times as many trees on rough The 3 inoculum sources were individual trees in
Table 2. Site, size, productivity, quality and survival of grapefruit trees carrying a strong strain of exocortis.
AverageW Average yield,,-t nu Soil tree _tyj>e Root stock __ (boxes (2) _Tree_s_uryival 1. Ruby Red 20 Wabasso' Rangpur 3.0 9.35 14/0/2 Rough lemon 3.4 8.73 14/0/6 Sour orange 2.9 9.94 14/0/0
2. Ruby Red 20 Park- Rangpur 2.7 9.62 6/0/0 wood Rough lemon 5.2 8.51 6/0/1 Sour orange 4.0 9.69 6/0/0
3. Marsh 10 Felda Rangpur 3.0 8.80 10/2/0 Rough lemon 5.8 7.86 10/0/1 Sour orange 3.3 9.21 10/0/0 (1> oldrtreesf * ^^ °f ^ ^ 10 ye" °1
Table 4. Scion, size, productivity, disease status and comments on some commercial plantings of citrus trees on 'Rangpur' lime rootstock on the east coast of Florida.
Foot Approx. Exocor- rot yield Grove tis approx. boxes/ # Scion Age Acres County strain % tree Remarks
1 Red Gft. 1 6 Ind . Riv. none 1 - Good growth
2 Marsh Gft. 6 0.5 Ind . Riv. none 1 - Good growth; medium quality 3 It ii it 6 80. none 33 good Large fruit; early maturity; low solids II 4 7 10. St. Lucie none 10 7.5 5 it ii 7(3) 6. none 1 3.0 Hamlin topworked with GF in 1967 If 6 ii ii 6 25. none 15 4.5 Frost damage; good size tt ii ii 7 3 30. none 5 8 It 5.5 45. Palm Beach none - good Early maturity; good growth in poor soil; solids O.K. 9 It 3.5 20. Ind, . Riv. none 10 - Good growth; low solids 10 It 5.5 7. Martin strong none 1.6 Bark scaling below bud union; good quality
11 Valencia 6. 10. Ind,. Riv. ? 5 2.5 12 ii it 1. 10. none 5 Good growth 13 ii 5. 15.. St. Lucie mild 5 1.0 Frost damage 14 it 3. 3. ii ti mild 1 - Good growth
15 Hamlin ii it 5. 100. none 8 1.5 Low solids; cold not a problem
it ii 16 Orlando 5. 20. ? 25 2.5
17 Robinson ii it 5. 20. none 10 2.0 Good performance
duce very harmful effects. This paper provides Field trials of 'Rangpur' lime rootstock in data showing that infected trees on 'Rangpur' commercial plantings on limited acreage can be are nevertheless often productive for their size, of 3 types: (1). Plantings with exocortis- and and that trees with exocortis may produce fruit xyloporosis-free scions as a replacement for of higher quality than virus-free trees. Trees in trees on rough lemon. Such trees can be ex experimental plantings also established a record pected to grow rapidly into large trees, to come of consistent bearing and good survival. into production early and to produce fruit with At present the indiscriminate use of exocortis- a low level of total soluble solids in the juice. inoculation of trees on 'Rangpur' would not be The apparent resistance of trees on 'Rangpur' to justified. Much remains to be learned regarding citrus blight suggests that it might be more the proper utilization of the exocortis-inoculation resistant also to young tree decline. It could technique. It is not yet clear whether it is the be used as a rootstock of value in a portion of exocortis per se or some accompanying constitu the areas affected by young tree decline. 'Rang ent which produces the effects observed. The pur' should not be used with virus-free scions use of mechanically-transmitted strains of of varieties which have difficulty meeting matur exocortis will help answer these questions. It ity standards, since it is no better than rough is not known whether the inoculum which is lemon under these conditions. (2). Plantings effective for one variety will be successful when with scions carrying exocortis and/or xyloporosis. used with other varieties. The effect of soil These should be plantings designed to reproduce types and environmental influences on the kind desirable existing virus-infected blocks like some of trees produced must also be investigated. For described in this article. They should be propa all these reasons it is best that new plantings gated from trees on 'Rangpur' lime in plantings on 'Rangpur' be small ones. Enough success has of this kind and should be set out on land of a been attained, however, to justify more trials in soil type similar to the original planting. (3). the immediate future. Plantings which are originally virus-free but are 84 FLORIDA STATE HORTICULTURAL SOCIETY, 1970 later inoculated with a strain of exocortis to sults of a trial initiated at Fort Pierce in 1950 on two soil types. Proc. Florida State Hort. Soc. 76:29-34. slow down growth. Plantings of this sort would 7. Cooper, William C, et al. 1956. Review of studies of adaptability of citrus varieties as rootstocks for grapefruit s^ow initial rapid growth but the growth rate in Texas. J. Rio Grande Valley Hort. Soc. 10:6-19. would slow down within a few years after 8. Creel, John M. Jr. and E. O. Olson. 1963. Cold damage to young navel orange trees on various rootstocks inoculation. Brix of the juice would increase in freezes of 1962 and 1963 in the Winter Garden area of Texas. J. Rio Grande Valley Hort. Soc. 17:55-59. following inoculation. This is similar to the plan 9. Ford, Harry W., 1964. The effect of rootstock, soil used for trees on P. trifoliata in Australia (1) type, and soil pH on citrus root growth in soils subject to flooding. Proc. Florida State Hort. Soc. 77:41-45. and for the experiment which provided the in 10. Gardner, F. E. and G. E. Horanic. 1958. Influence of various rootstocks on the cold resistance of the scion va formation or Table 3. Results with this type of riety. Proc. Fla. State Hort. Soc. 71:81-86. field trial will, at first, be the least predictable 11. Gardner, F. E. and George E. Horanic. 1963. Cold tolerance and vigor of young citrus trees on various root of the 3 mentioned here but experiments of this stocks. Proc. Fla. State Hort. Soc. 76: 105-110. 12. Mendel, K. 1969. New concepts in stionic relations kind will eventually provide trees most closely of citrus, p. 387-390. In Homer D. Chapman (ed.) Proc. designed to fit a given situation. First Intern. Citrus Symposium. Vol. 1. University of Cali fornia, Riverside. 13. Moreira, S. 1964. Citrus virus diseases. FAO Plant Protection Bulletin. 12:57-66. 14. Olson, E. 0. 1952. Investigations of citrus root- LITERATURE CITED stock diseases in Texas. Proc. Rio Grande Valley Hort. Inst. 6:28-34. 1. Anonymous. 1966. Dwarf orange trees. The Agric. 15. Olson, E. O., and A. V. Shull. 1962. Size and yield Gazette of New South Wales. 77:561-562. of 12-year-old 'Valencia' orange trees on various rootstocks 2. Benton, R. J., F. T. Bowman, L. Fraser and R. G. in the presence or absence of exocortis and xyloporosis vi Kebby. 1949-50. Stunting and scaly butt of citrus associated ruses. J. Rio Grande Valley Hort. Soc. 16:40-43. with Poncirus trifoliata rootstock. Agric. Gazette of New 16. Phillips, R. L. 1969. Dwarfing rootstocks for citrus, South Wales. 60:521-526, 577-582, 649-654; 61: 20-22, 40. p. 401-406. In Homer D. Chapman (ed.) Proc. First Intern. 3. Calavan, E. C, et al. 1964. Rapid indexing for exo Citrus Symposium. Vol. 1. University of California, River cortis of citrus. Phytopathology. 54:1359-1362. side. 4. Calavan, E. C, et al. 1968. Effect of exocortis on pro 17. Reitz, H. J. and L. C. Knorr. 1957. Occurrence of duction and growth of Valencia orange trees on trifoliate •Rangpur' lime disease in Florida and its occurrence with orange rootstock. p. 101-104. In J. F. Childs (ed.) Proc. 4th exocortis. Plant Disease Reptr. 41:235-240. Conf. Intern. Organization Citrus Virol. Univ. Florida 18. Reuther, Walter, Herbert John Webber and Leon Press, Gainesville. Dexter Batchelor (Eds.). 1967. The Citrus Industry, Vol. 1. 5. Cohen, Mortimer. 1968. Exocortis virus as a possible Revised edition. University of Calif. Division of Agric. factor in producing dwarf citrus trees. Proc. Florida State Sciences. P. 532. Hort. Soc. 81:115-119. 19. Sinclair, J. B. and R. T. Brown. 1960. Effect of exo 6. Cohen, Mortimer and Herman J. Reitz. 1963. Root- cortis disease on four citrus rootstocks. Plant Disease Reptr. stocks for Valencia orange and Ruby red grapefruit: re 44:180-183.
PROBLEMS IN USING 'MILAM' ROOTSTOCK AS A BIOLOGICAL BARRIER
Harry W. Ford in conjunction with a narrow root-killing chemi cal barrier* This is necessary to minimize the Florida Citrus Experiment Station spread of burrowing nematodes on susceptible Lake Alfred roots growing into the biological barrier area. The chemical barrier should be renewed when Abstract there is evidence (by digging into the chemical barrier area with a shovel) that roots of 'Milam' 'Milam' rootstock has not been supporting and susceptible hosts are intermingling. This populations of burrowing nematodes in biological period may be as long as 2 years. barrier tests in the field. However, the growth 'Milam' should not be planted in nonfumi- of rough lemon roots into the barrier area can gated soil because of the added danger of infesta harbor and enhance the migration of burrowing tions of citrus nematodes and meadow nematodes. nematodes and other pathogenic nematodes such as citrus nematodes to which 'Milam' may be susceptible. Greenhouse Studies 'Milam' as a biological barrier must be used 'Milam' in 2 separate tests has been evalu ated for 6 and 7 years respectively as a biological Florida Agricultural Experiment Station Journal Series barrier, utilizing large soil tanks 25 feet in No. 3720.